you can get a hardwood front door in standard sizes (about 32" width) readymade in fair quality for a few hundred pounds. It is worth getting a new frame unles the old one is good quality and condition.
Look for mortice and tenon joints; solid hardwood (not veneer on laminated rubbish core). The DIY sheds are not usually good quality. If you pick up a good door you will find it hard work as they are very heavy.
the joinery manufacturers websites will have a selection.
A stable door is quite handy if you have small children in the house as you can stop them getting to the road but have the top open for fresh air, chatting to neighbours etc. Real stables to not have a any glazing, open out, and are usually LB&F.
My front foor (not stable) is h/w M&T, I went for thick timber not the thin glazing bars as IMMO they are easier to break in. If your friends are spending a wedge of money they might consider laminated glass for any glazing as well, it is much more secure.
There may be a local maker near you as well. Look up "joinery" or "joiners" if they want it fitted and ask the joiner for a recommendation. Include locks, bolts, brass hinges in the price. DIY hanging is slow and hard unless you are skilled. Paint or treat before fitting, at least an undercoat, as some parts are very difficult to do after (it will want refinishing afterwards due to scratches but ask the joiner if you can paint the bottom at least (this is impossible to treat after fitting) without him cutting it off